ICAO markings are used here, are they not? Curiously, those red boxes are called "Mandatory instruction markings" but the MOS says "may" and "should" regarding their use.
Because the "Mandatory" is not pertaining to their installation. It refers to the information they portray. They "may" (there's no "should" that I can see) be used as an adjunct to MAGS that display Mandatory Instructions (as opposed to ones that are Information Only ones). The MAGS have rules as to where and why they are installed. (Section 8.79 (3) & (4))